According to FIG. 1, previous knee prosthesis are oriented on the so-called mechanical axis δ1 with respect to its joint line 2′. The mechanical axis δ1 is the line that runs vertically from the center 1 of the head of the femur through the knee joint 3 to the ankle 4. The joint lines 2′ and 5′ of previous knee prosthesis implants were implanted at an angle of (β1) 90° to this mechanical axis. The medial and lateral condyles of the femur were equally large in regard to their polycentric radius.
In reality, it is now the case that the joint line 5 of the upper ankle joint 4 to the axis δ1 forms an angle (α1) of 85°±5° ascending medially and descending laterally. Furthermore, in the region of the knee joint line 2, the anatomical healthy knee joint also does not form a right angle with the mechanical axis δ1 that was assumed at that time, but rather the tibial plateau descends medially by an angle (β2) of about 3°±x or ascends laterally by 3°±x relative to the mechanical axis δ1. The consequence of this is that the condyles of the femur have different radii. Laterally, the polycentric radii are somewhat smaller in the side view, while the medial condyle of the femur is distinctively larger with polycentric radii, as far as the radial diameter is concerned. The bearing surface in the a.p. plane is somewhat narrower in the case of the medial condyle of the femur, compared to the lateral condylar part. In addition, it should be noted that the tibial plateau drops by 3-7° in the dorsal direction.
Now, since the knee joint carries out an internal rotation in the course of knee flexion, the medial part of the femoral condylar structure moves ventrally (uphill, so to speak, with respect to the tibial plateau), whereas the lateral part of the condylar structure of the femur moves dorsally and thus downhill, so to speak. The 3° axis in the knee joint (towards the old mechanical axis δ1), which is already present anyway, is thus indirectly enlarged, so that the tibial plateau no longer forms an angle of 3° with the original old mechanical axis δ1, but rather now forms an angle of 3°+x, which then corresponds approximately to the angle of the upper ankle joint in relation to the former mechanical axis, i.e., generally about 5°.
Of course, independently of this, these polycentric radii also have an effect on the point of attachment of the collateral ligaments, which must be continually under tension, such that by changing the anatomical conditions by creating two equally large femoral condyles and a horizontal tibial plateau, conditions that differ completely from the anatomy are created here for the collateral ligaments as well. For this reason, the further inventive step of developing an anatomical knee prosthesis is taken here, which is intended to take these anatomical conditions exactly into account. All of the problems are solved by the features specified in the claims.